If he is smart, he will wait it out as long as needed until the perfect opportunity comes up. He's got a QB who hasn't even hit his prime yet and plenty of young offensive weapons. No reason to think his stock won't continue to be high every year as long as he has these players healthy
Sure, being a HC is always the goal but is it worth it to go start your coaching career on a bad team like the Lions, Jags, Jets, etc.? If he goes to a really bad team with a poor outlook and bad ownership/front office then he will just be destined to fail.
A lot of successful coaches don't reach their potential until they get fired from their first team (ex: Belichick, Dungy, Coughlin, Reid, Carroll, Arians). Actually, it might not be so bad if he left now and went to get experience/fail with another team. Bring him back in 4 years!